Method for producing an iron melt using iron-containing residual smelting plant materials

ABSTRACT

For effectively reprocessing iron-containing residual smelting plant materials ( 1  to  3 ), in which iron may be present both in metallic form and in oxidic form, with lowest possible energy expenditure, the residual smelting plant materials ( 1  to  3 ) are processed into agglomerates ( 8,11 ), the agglomerates ( 8,11 ) are charged into an electric arc furnace ( 10 ), melted there and reduced, and the resultant melt is refined (FIG.  1 ).

The invention concerns a method for producing an iron melt, inparticular a crude steel melt, using iron-containing residual smeltingplant materials, and an installation for carrying out the method.

For a process of directly reducing iron ore with subsequent melting downof iron sponge and simultaneous coal gasification, it is known(AT-B-376,241) to separate particulate solids, primarily consisting ofcarbon in dust form, from the reduction gas formed in a fusiongasification zone and from the waste gas produced during directreduction, to mix the separated particulate solids with binder,including with iron oxide dust, to make formed coke by hot-briquettingand subsequently to feed the formed coke back to the melting-downprocess.

It is disadvantageous in this case, however, that, because of theintroduction of iron oxides in the fusion gasifier, reduction work hasto be performed in order to reduce the iron oxide, whereby energyrequired for the melting-down process is taken from the said iron oxideand the process taking place in the fusion gasification zone isdisrupted. Furthermore, the said hot-briquetting represents an expensivesolution with regard to the investment and operating costs.

It is known from DE-A-41 23 626 to agglomerate residual smelting plantmaterials of mixed consistency, with the aid of binders, slag-formingconstituents and reduction agents, and to introduce the agglomeratesinto the upper burden region of a melting unit, the preheating anddrying of the agglomerates taking place in this burden region of themelting unit. The burden passes through the melting unit on the basis ofthe counterflow principle, it initially arriving in a reduction region,provided in the interior of the melting unit, and being subsequentlymelted in the lower region of the melting unit.

This known method is energy-intensive to the extent that metallic scrapor residual materials also have to pass through the reduction region ofthe melting unit. A particular problem here is the stability of theagglomerates, since these agglomerates are used while still in the greenstate, that is to say not completely dried, which in practice causesgreat difficulties due to disintegration, abrasion etc. When passingthrough the melting unit on the basis of the counterflow principle,destruction of the agglomerates caused by forces of pressure and impactcan lead to a high proportion of the said agglomerates being dischargedfrom the melting unit through the waste gas. For this reason, the methodknown from DE-A-41 23 626 can only be realized with difficulty inpractice. Agglomerates with a high stability, which they should alsohave in the high temperature range, would have to be produced, which inturn is very expensive, however, and would require in particular the useof high-grade and correspondingly expensive binders.

It is known from AT-B-380,901 to convey metal-oxide-containingmetallurgical dusts together with carbon-containing material through arotary tube, to reduce them in a hot zone of the tube and to form ironsponge, and to use this iron sponge in a converter as a substitute forchill scrap. This method has proven successful in the case ofmetal-oxide-containing metallurgical dusts, but requires additionalexpenditure on apparatus and process technology to establish a reducingatmosphere. What is more, only metal-oxide-containing metallurgicaldusts can be reprocessed; this method is not envisaged for the use ofdusts containing high proportions of metallic iron.

A method of the type described at the beginning is likewise known fromEP-A-0 623 684. In this case, a complete and energy-saving reprocessingof waste and residual materials of the metallurgical industry issuccessfully achieved in a method for the direct reduction of iron oreinto iron sponge and melting down of the iron sponge in a coalgasification zone, it being necessary however to collect the waste orresidual materials separately in groups according to their chemicalcomposition. A first group mainly comprises iron in oxidic form, asecond group iron in metallic form and a third group mainlycarbon-containing materials. The first group is charged into the directreduction zone and the second and third groups are charged directly intothe fusion gasification zone, charging being preceded by carrying outthickening and granulating of the waste and residual materials occurringin the form of slurries.

In the case of this known method, it is primarily intended to feed thedusts produced in the waste gases during direct reduction ormelting-down and during coal gasification back to the direct reductionor the melting-down process and coal-gasification process. This iscomplex to the extent that the agglomerates are in turn fed back to thesame processes from which they originate. They consequently have to beheated up again there and run through these processes as it were asecond time, and only subsequently can they be further processed in adownstream process, for example a process for producing steel from pigiron.

It is likewise know from U.S. Pat. No. 5,100,464 to mix residualsmelting plant materials, bind them with molasses and to make briquettesin a cold-briquetting process and charge them into a converter.

A method of this type is also know from WO-A-96/34120 as well as U.S.Pat. No. Re. 30,795 and U.S. Pat. No. 4,119,455.

Furthermore, numerous methods of melting down zinc-containingmetallurgical dusts are known, in which zinc-containing dusts are melteddown by means of electrical energy, such as by means of plasma burnersor conventional electric arcs. These methods serve for recovering thezinc, but not for producing an iron melt. These methods are known by thenames “Mintek method”, “Elkem method”, “IMS-Tectronics method” or “DavyMcKee method”.

It is generally known in the blast-furnace process, oxygen steelmakingprocess or in the direct reduction of iron ore into iron sponge toseparate dusts occurring in the wet process from the waste gases formingduring these processes, and to dry the slurries thereby formed, but theslurries are usually subsequently landfilled for reasons of low cost.This has been accepted in the past, since these slurries (as drysubstance) only make up approximately 1.5% by weight of the amount ofsteel produced. However, with increasing environmental awareness, thereis increasingly a requirement to avoid such landfill sites. Thisinvolves difficulties, however, since—as explained above—reprocessing ofthe dusts in the metallurgical industry currently requires great effort,such as selection, and the dusts often have to be discharged in turnwith waste gases.

The invention is based on the object of effectively reprocessingiron-containing residual smelting plant materials, in which iron may bepresent both in metallic form and in oxidic form, with lowest possibleenergy expenditure and with an expenditure on apparatus requiring onlylow investments, to be specific involving recovery of the iron containedin these residual smelting plant materials and wherever possibleallowing technologies successfully proven in practice to be used. Inparticular, the intention is to avoid the dusts passing repeatedlythrough process stages provided one after the other in steel productionand avoid them having an additional adverse effect on these stages.

This object is achieved according to the invention by the combination offollowing features:

residual smelting plant materials are processed into agglomerates,

the agglomerates are charged into an electric arc furnace,

melted and reduced,

and the resultant melt is refined.

It is of particular advantage if liquid and/or solid pig iron isadditionally charged into the electric arc furnace and is likewiserefined, the pig iron expediently being at least partially chargedbefore the residual smelting plant materials. For energy-relatedreasons, the pig iron is charged in the liquid state. However, it mayalso be introduced in the form of ingots, either entirely or only inpart. Part of the pig iron may also be substituted by scrap. There isalso the possibility of substituting pig iron by a carburized liquidpool remaining from the residual melt. Carburizing may take place byadding lump and/or dust coal/coke.

With the method according to the invention, residual smelting plantmaterials can be successfully processed in large amounts. Residualsmelting plant materials are advantageously charged in an amount ofpreferably approximately 40 to 50% of the total charge.

As the metal product, a crude steel, a semisteel or liquid pig iron canbe produced.

However, in regular operation, i.e. with residual smelting plantmaterials occurring in normal amounts, refining is advantageouslycarried out up to a carbon content of at most 0.1%, i.e. the metalproduct is a crude steel.

The method according to the invention is also very well suited forprocessing rolling scale slurry, this advantageously having the oilremoved beforehand, by treating with specific lime additives, such ascalcined lime. This measure allows the oil contained in the rollingscale slurry to be chemically dispersed by CaO, before a hydrationreaction with water occurs.

Dusts and/or slurries from converter and/or electric arc-furnaceoperation and/or the pig-iron production installation and/or the directreduction installation are used in particular for charging as residualsmelting plant material.

The residual smelting plant materials are advantageously mixed beforecharging and have calcined lime added as a binder, whereupon theresidual smelting plant materials are agglomerated and the agglomeratesare dried to a residual moisture content of less than 5%.

Preferably, first of all pig iron, preferably liquid pig iron, ischarged into the electric arc furnace, whereupon the residual smeltingplant materials are continuously charged over a predetermined timeperiod and, during this time, the refining process is carried out,refining expediently taking place for a predetermined remaining time,for the vaporizing of zinc, without any charging of residual smeltingmaterials being carried out.

An installation for carrying out the method according to the inventionis designed as follows: with a mixing reactor, into which there opens atleast one line feeding in residual smelting plant materials, an H₂O feedline and a line feeding in binder, with an agglomerating device,preferably with a drying device, from which the dried agglomerates canbe passed to a preferably provided screening device, with an electricarc furnace, into which there leads at least one line feeding in theagglomerates, preferably with a line passing the coarse fraction fromthe screening device into the electric arc furnace through at least onecover opening and/or a line passing the fine fraction to at least onelance protruding into the electric arc furnace, with a feed for pig ironleading into the electric arc furnace, a tapping opening for the slagand a tapping opening for the iron melt produced.

The installation preferably has a filter system, to which there leads awaste-gas line, extending from the arc furnace, and from which filtersystem a line for dusts separated from the waste gas leads to the mixingreactor, there expediently extending from the drying device a linecarrying away vapours, which leads to the filter system.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below on the basis of anexemplary embodiment represented in the drawing.

FIG. 1 illustrating the method according to the invention in a schematicflow-sheet representation.

FIG. 2 reproduces the process sequence in the electric arc furnace foran exemplary embodiment.

FIG. 3 shows a section through an electric furnace, as used in the caseof the method according to the invention, and

FIG. 4 is a plan view of this furnace in the direction of the arrow IVof FIG. 3.

DESCRIPTION OF THE DRAWINGS

The flow sheet represented in FIG. 1 is subdivided into four areas,which are denoted by I to IV. Listed in area I are the charged materialsfor carrying out the method according to the invention. Area II concernsthe agglomeration and, area III the post-treatment after theagglomeration, before charging into the electric arc furnace. Field IVconcerns the waste-gas treatment.

The method according to the invention is suitable for the disposal ofall residual smelting plant materials, in particular even oil-containingrolling scale slurry 1, as well as blast-furnace slurry 2 and furtherdusts 3 from converter operation and from electric arc-furnaceoperation. These residual smelting plant materials are fed to a mixingreactor 4, in which they are thoroughly mixed, with the addition ofwater. The oil-containing rolling scale slurry 1 is—if necessary—treatedbeforehand with calcined lime, whereby the oil is chemically dispersedbefore a hydration reaction with water occurs. This has the effect offorming a pourable powder, in which the oil is firmly bound. Thismeasure allows the rolling scale slurry 1 to be further processedtogether with the dusts 3 from converter operation or electricarc-furnace operation and the blast-furnace slurry 2. If appropriate, ifit contains a coarse fraction, the powder formed by the dispersion canbe separated into a dust fraction and a coarse fraction, the coarsefraction being ground before mixing in with the other residual smeltingplant materials 2, 3.

In the mixing reactor 4 there takes place in addition to the mixing ofthe residual smelting plant materials 1 to 3 also the mixing in ofcalcined lime 5 as a binder, calcined lime 5 preferably being added inan amount equivalent to between 5 and 10%. The moisture content isadjusted by the addition of water to approximately 15% by weight.

As soon as the mixture 6 has reacted, it is transferred into the mixinggranulator 7, in which green pellets 8 are formed. Arranged downstreamof this mixing granulator 7 there may be a Rollier drum, not shown inthe drawing, if green pellets 8 of a specific minimum size are required.The green pellets 8 expediently have a diameter of between 5 and 10 mmand have a moisture content of approximately 15%.

The granulation is followed by a drying of the green pellets,advantageously in an oscillating dryer 9, to a moisture content of <5%.As a result, the green pellets 6 are adequately stable for charging intoan electric arc furnace 10. If an oscillating dryer 9 is used, hot airis passed through the flow-applying base and the product layer duringthe oscillating transport of the green pellets 6 in the direction of thedischarge opening of the oscillating dryer, whereby the green pellets 6are dried to the required maximum moisture content. The air fed to theoscillating dryer is preheated by means of a natural-gas burner system.

The dried pellets 11 are subsequently separated into two screeningfractions 11′, 11″, expediently by an oscillating screening, a coarsefraction 11″ of the pellets with a particle size of over 5 mm and a finefraction 11′ with pellets of up to 5 mm being formed. The two fractions11′, 11″ are subsequently charged into the electric arc furnace 10, thecoarse fraction 11″ via at least one cover hole 12 and the fine fraction11′ via lances 13. The ratios of the amounts of the fine fraction 11′and coarse fraction 11″ to be charged may be flexible within a widerange for optimum melting operation in the electric arc furnace 10.

The electric arc furnace 10 may be operated both as a DC furnace or asan AC furnace. In the case of the exemplary embodiment represented inFIGS. 3 and 4, it is operated on alternating current. Three graphiteelectrodes 14 protrude through the cover 15 into the interior. Thefurnace 10 is provided with a cover opening 12 for feeding in the coarsefraction 11″ of the residual smelting plant materials 1 to 3 andadditives, such as lime, dolomite and other slag-forming constituents aswell as lump coal or coke etc.

The fine fraction 11′ is introduced—as mentioned—by lances 13 to whichair is admitted. In addition, lances 16 are also provided for feeding inoxygen and/or an oxygen-containing gas, such as air. For achievingintensive bath mixing, the electric arc furnace (10) is to be providedwith bottom flushing elements, preferably with bottom nozzles 17, withpreferably inert gas. Oxygen and coal are introduced into the electricarc furnace 10 by means of a manipulator 18 through the slag door 19.

For carburizing the metal bath, the electric arc furnace (10) may beprovided with at least one carbon under-bath nozzle.

The crude steel tap 20 is located on the side of the electric arcfurnace 10 opposite the slag tapping opening 19. The waste gas formed inthe electric arc furnace is fed via an elbow 21, in a way still to bedescribed later, to the waste-gas post-treatment represented in area IVof FIG. 1.

Introduced first of all into the electric arc furnace is pig iron 22,for example from a blast furnace, preferably in an amount of between 50and 60% of the total charge. The residual smelting plant materials 1 to3, that is to say the fine fraction 11′ and coarse fraction 11″, aresubsequently charged into the electric arc furnace 10 over a certaintime interval (preferably approximately 60 min). During this time,refining with oxygen is simultaneously carried out. Subsequently, onlyrefining is carried out for a certain time interval, without theaddition of residual smelting plant materials 1 to 3, in order for thezinc to be largely vaporized. The tap-to-tap time is approximately 90 to100 min.

The residual smelting plant materials 1 to 3 generally have a carboncontent of approximately 7% by weight, so that when operating withapproximately 60% pig iron 22 and 40% residual smelting plant materials1 to 3, scarcely any additional coal is required for reduction. Whenoperating with foamed slag, the slag tapping takes place continuouslyafter half the tap-to-tap time, approximately 140 kg of slag 24 beingproduced per tonne of crude steel 23. There are no objections to theslag 24 being reprocessed for building materials, on account of thechemical composition of the slag.

A completely refined high-grade crude steel 23 with less than 0.1%carbon is preferably produced in the electric arc furnace 10.

Typical properties of the crude steel 23:

0.05% C 0.08% Mn 0.016% P 0.073% S tapping temperature 1650° C.

A desulphurization takes place during a subsequentsecondary-metallurgical treatment. This secondary-metallurgicaltreatment advantageously takes place in a ladel furnace 25.

The waste gases from the drying of the green pellets 8 and from theelectric arc furnace 10 are fed to a filter system 26, from which thedust 27 occurring is recycled into the reconditioning process for theresidual smelting plant materials.

In order to expel the concentrating components (e.g. Zn, Pb etc.), apartial stream 29 may be expelled if it is ascertained by means oflaser-optical measurement 28 that a certain content of these componentsin the dust is exceeded. This partial stream 29 may either be furtherconcentrated or transferred directly to further processors (e.g. zincsmelters).

The method according to the invention has major advantages over theconventional methods:

Considerable proceeds can be earned by processing the residual smeltingplant material into an iron-containing melt.

The electric arc furnace 10 used shows great flexibility in the chargeratio of pig iron 24/residual smelting plant materials 1 to 3;consequently, it is easy to respond to fluctuating amounts andcompositions of residual smelting plant materials.

According to the wishes of the operator, a refined crude steel 23, asemisteel product or liquid pig iron can be produced.

The dust 27 emitted by the electric arc furnace 10 is recirculated forzinc enrichment until a certain zinc content is obtained. The high zincenrichment allows the amounts of residual material occurring to beminimized.

In the conditioning of the residual smelting plant materials 1 to 3, noorganic substances are introduced via the binders 5.

The high temperature prevents process-related zinc or lead condensationin the electric arc furnace 10. Furthermore, there are no problems withthe furnace lining, since the process operates with foamed slag.

At the optimum charge ratio of the charge materials, the slag 24 can bereprocessed as building material. Consequently, no additional landfillcosts arise.

The investment costs of the electric arc furnace 10 are lower incomparison with a shaft furnace.

Quick starting up and shutting down of the electric arc furnace 10presents no problem.

By contrast, a method using a shaft furnace has the disadvantages thatquick starting up and shutting down of a shaft furnace is not possible,that a far greater stability of the granulated material, i.e. thepellets, is required, and sometimes cannot be achieved at all, and thattemperatures of over 950° C. become necessary to deal with the zinccondensate problem in the shaft outlet zones.

Exemplary Embodiment

Oil-containing rolling scale 1, which has been pretreated with anadditive, as well as blast-furnace top gas mud 2, converter dust 3 andlime are introduced into the mixing reactor. Calcined lime is envisagedas the additive for the oil-containing rolling scale 1. The quantitativeratios are reproduced in the following Table I.

TABLE I oil-containing 6.9 t/h 8.3 t/h rolling scale 1 50,000 t/a 60,000t/a calcined lime 1.4 t/h additive 10,000 t/a blast-furnace top 6.9 t/hgas mud 2 50,000 t/a converter dust 3 25.0 t/h 180,000 t/a lime 3.0 t/h20,000 t/a

Altogether, 44.1 t/h or 317,200 t/a are introduced into the mixingreactor 4. Addition of water at a rate of 1 t/h or 7,200 t/a isrequired. After forming the granules or pellets 8 and transferring thesame into the dryer 9, the screening takes place, producing 40 t/h, i.e.288,000 t/a, of pellets 11, to be precise 19 t/h with a fine fraction11′ and 21 t/h of a coarse fraction 11″. That is 136,800 t/a of finefraction 11′ and 151,200 t/a of coarse fraction 11″. 4.1 t/h of vapours30 are formed in the dryer, that is 29,200 t/a. The vapours 30 arelikewise passed to the filter system 26.

This fine fraction 11′ and coarse fraction 11″ is charged into theelectric arc furnace 10, which is operated with a charge mix of 60%liquid pig iron 22 and 40% residual smelting plant materials 1, 2, 3.For each tonne of crude steel 23 produced in the electric arc furnace10, 278 kg of pellets of over 5 mm (coarse fraction 11″) are fed in viathe cover 15 together with 14.3 kg of lump lime and 14.2 kg of lumpdolomite and 250 kg of fine fraction 11′, that is to say pellets smallerthan 5 mm, are fed in with the aid of 25 Nm³ of compressed air via twolances 13. 26.4 Nm³ of oxygen and 1.7 kg of blasting coal are introducedinto the electric arc furnace 10 by means of the manipulator 18. Theliquid pig iron 22 is charged in a liquid state at the beginning of theprocess into the electric arc furnace in an amount of 791 kg, the pigiron 22 containing

4.3% carbon,

0.6% silicon,

0.5% manganese,

0.09% phosphorus and

0.005% sulphur,

the remainder being iron.

The pig iron 22 has a temperature of 1320° C. 0.5 NM³ of N₂ and 0.5 Nm³of CH₄ are introduced via the bottom nozzles 17 for bath mixing. 14.7Nm³ of O₂ are fed in via three post-combustion lances just underneaththe slag surface, so that a partial CO+H₂ post-combustion takes placefrom the primary-produced furnace waste gas, and the resultant energy istransferred efficiently to the metal bath.

About 83 Nm³ of infiltrated air likewise get into the electric arcfurnace 10. With 500 kWh of electrical energy supplied, 143 kg of slag24 with the composition specified in Table II form in the said furnace.

TABLE II 27.9% FeO_(n) about 5% Fe_(met) CaO/SiO₂ =2.2 7.1% MgO <5.4%Na₂O 1.27% P₂O₅ 0.42% S

1000 kg of crude steel 23 with the chemical composition specified inTable III are tapped. The temperature of the crude steel is 1650° C.

TABLE III 0.05% C 0.08% Mn 0.016% P 0.073% S 120 ppm Zn 30 ppm Pb 40 ppmN

The process sequence for the exemplary embodiment described above can betaken from FIG. 2.

What is claimed is:
 1. Method for producing an iron melt using ironcontaining residual smelting plant material characterized by thecombination of the following features: processing residual plantmaterials into agglomerates, charging the agglomerates into an electricarc furnace being operated with foamed slag, melting and reducing theagglomerates in a pig iron melt, thereby producing a further melt, andrefining the further melt.
 2. Method according to claim 1, characterizedin additionally charging the electric arc furnace with liquid or solidor liquid and solid pig iron, which is likewise refined.
 3. Methodaccording to claim 2, characterized in charging the pig iron at leastpartially before the residual smelting plant materials.
 4. Methodaccording to claim 3, characterized in that the pig iron is substitutedpartially or entirely by a carburized residual liquid pool.
 5. Methodaccording to claim 2, characterized in that residual smelting plantmaterials are charged in an amount of at least 5% of the total charge.6. Method according to claim 5, characterized in that residual smeltingplant materials are charged in an amount of approximately 40 to 50% ofthe total charge.
 7. Method according to claim 1, characterized in thatcrude steel, semisteel or liquid pig iron is produced as the metalproduct.
 8. Method according to claim 1, characterized in that a crudesteel with a carbon content of at most 0.1 weight % is produced. 9.Method according to claim 1, characterized in that rolling scale slurryis charged as the residual smelting plant material.
 10. Method accordingto claim 1, characterized in that dusts or slurries or both from atleast one of a converter, the electric arc furnace, a pig ironproduction installation and a direct reduction installation are chargedas residual smelting plant material.
 11. Method according to claim 1,characterized in that the residual smelting plant materials are mixedbefore charging and have a calcined lime (CaO) binder, whereupon theresidual smelting plant material are agglomerated and the agglomeratesthus formed are dried to a residual moisture content of less than 5%.12. Method according to claim 1, characterized in that first of all pigiron is charged into the electric arc furnace and then the residualsmelting plant materials are continuously charged over a period of timeand, during this time, the refining process is carried out.
 13. Methodaccording to claim 1, characterized in that refining takes place for atime without any charging of residual smelting plant materials beingcarried out.